Beam loaded in two axes 1

[conradlovejoy]

I hope I am not thinking about this situation erroneously but I want to know if there is a combined effect for a beam that will be loaded in two axes. The situation I am dealing with involves a cabana structure with a gable roof and half height walls. One of the methods I came up with for framing the roof involves rafters with a non-structural ridge board, resulting in outward thrust from the rafters on the eave beams. Can the eave beams be designed in one axis at a time (flexure, shear, and deflection checks about the "gravity" axis as well as f,v,d checks about the thrust axis; worst case controls) or is there some combined loading effect that I am neglecting?

 

[hokie66]

If the loading in two directions occurs concurrently, you have to consider the combined effect. That would be the case for roof beams.

 

[conradlovejoy]

Unfortunately, I can only think of cases where combined loading means axial and bending occurring, simultaneously. In this case, the same type of stress is applied to two different axes. I don't recall ever needing to calculate forces in that manner. Do you have an example that wouldn't be too tedious to describe?

 

[dhengr]

Conradlovejoy:
What does the word ‘superposition’ mean to you? Look it up. The gravity loads are assumed to be applied to the top of the beam and through its centroid. The trust load is applied at the top of the beam and imparts lateral (weak axis) bending, shear and some torsional component. You analyze the beam for all of these load conditions and then combine the stresses, using the correct sign conventions to get the max. combined stresses to see if the beam passes muster. You should have tie beams (ceiling joists?) on at least ever other rafter pair, and this would eliminate the thrust loading on the beams.

 

[conradlovejoy]

Thanks. I understand the concept of superpostion. I just didn't realize the stresses would be additive in this case but that makes sense.

I am in the prelim stages right now so it is sort of up to me. However, there is no ceiling (therefore no ceiling joists) because the arch wants a open/vaulted look to the cabana. I know there are other ways to resist the thrust force such as steel cable ties and whatnot, but in the even the arch doesn't want any of that, I just wanted to be prepared and see if I could get an eave beam to carry it all and stay within the "look" the arch desires.

 

[dhengr]

Conradlovejoy:
Architects get all crazy that way, they gaze off into the distance and imagine the great edifice to their egos which they are birthing, and all loads just float away into thin air. Then they are off the pick the other pink tile or granite counter tops, so there’s no money left for the structure. But, you have to make the damn thing stand up under all code conditions, all likely applied loads. And, they do still want you there as the insurer of last resort when something you said shouldn’t be done starts going south. Don’t forget, that the thrust goes to the top of the beam and then gets carried down to the columns and the footings which must cantilever up to take that trust. There can be other lateral loads on this structure as well as uplift on the roof structure, so give some thought to how your are going to brace it laterally in both directions. You could use tie beams (joists?) a couple feet above the rafter bearing/top of beam elev., but if they are much higher than that they become much less effective as they load the rafters too much in bending. You could use scissors trusses, but they still have a lateral thrust or lateral deflection at their bearings. Then, put a nice pine car siding on the bottom chord and you have your vaulted ceiling, but vent the attic space. You could use light timber trusses dolled up a bit, wrought iron hardware, etc., at 4, 6 or 8' o/c, for a timber frame look. But, the upshot is that you can’t put a roof system 8-9' brg. elev. with a15-16' high ridge, up in the air on six or eight toothpicks and not have trouble.